Avoid applying equalization, panning or effects, such as reverb or chorus, to a vocal track prior to layering. Using a dry copy of the vocals when layering will leave more creative options open later.
Set up your multitrack recording for playback. For example, if you are using a software mixer, load the file containing your track and open the mixer window. If you are using a hardware mixer, set up your ADAT or other recording device to play back each track through its own channel strip on the mixer.
Isolate the vocal track you want to layer. In most cases, this is done by engaging the solo function associated with the vocal track. For example, most software mixers include a "Solo" or "S" button on the track header for each track. Similarly, most hardware mixers will have a labeled "Solo" or "S" button somewhere on each channel strip. Engaging the solo function will cause the vocal track to play back alone.
Create one or more copies of the vocal track and place them on their own tracks. In most software mixers, there will be an option to duplicate the selected track. In the hardware realm, there are numerous options for duplicating a track. Many ADAT recorders include an option to automatically create a copy on an unoccupied track, for example. Consult your hardware's documentation for details.
Engage the solo function on each track containing a copy of the vocal track. This will cause them to play back together with the original. The basic layering process is complete at this point. However, if you press "Play," you are unlikely to hear much of a difference in sound. This is because the tracks all contain exactly the same sound information. The purpose of layering is to manipulate each copy in a slightly different way for different effects.
Use panning to give the layered vocal track more width in the mix. For example, leave the original vocal track center-panned, then pan one copy left and another copy right. Most professionally produced lead vocal tracks are layered and panned this way to help the vocals properly dominate the mix.
Use equalization on the different layers to achieve a wide range of tonal effects. For example, emphasize one frequency range on one copy and another frequency range on another copy. This is often done with lead vocals to help them avoid being drowned out by other sounds with a similar overall frequency range, such as a trumpet or saxophone section.
Experiment with different reverb, chorus or other audio effects on the different layers. For example, reverb is sometimes applied only to one copy of the vocal track to avoid reverb saturation, which can muddy the mix. Above all, this stage of the mixing process is a creative process. Layering has practical applications, but is also a powerful creative tool in digital audio production.